The present invention relates generally to apparatus and methods for fracturing subterranean oil and gas producing formations to stimulate production fluid recovery therefrom. In a preferred embodiment thereof, the present invention more particularly provides improved apparatus and methods for forming casing perforation sealing structures and positioning perforation guns for subsequent casing perforation and earth fracturing at progressively higher levels along a well casing.
The general process of hydraulically fracturing vertically spaced zones of subterranean oil and gas producing formations, through spaced series of well casing perforation areas, to stimulate production fluid recovery is widely known and utilized in various forms. A conventional method comprises the lowering, on a wireline, of an explosive perforating gun containing shaped charges to a predetermined depth within a fracturing fluid-filled well casing. Electric detonation of the gun creates perforations in the casing through which the frac fluid is outwardly forced, by surface pumping equipment, to hydraulically fracture the adjacent subterranean formation.
As illustrated and described in U.S. Pat. Nos. 4,633,951 and 4,718,493, this general perforation and fracturing technique has been substantially improved via the incorporation of a "foam decompression fracturing" (FDF) process in which a gas is injected into the frac fluid to foam and highly pressurize it prior to detonation of the perforating gun. After the gun is fired, the highly pressurized frac foam exits the resulting casing perforations at near sonic velocity, releasing its great amount of stored compressive energy to greatly facilitate the fracturing of the adjacent subterranean formation.
After this initial fracture zone has been formed, by one of the above-described methods, the fractures therein are "propped" with a quantity of proppant fluid flowed outwardly through the casing perforations, and a "sand-off" operation is performed to plug the perforations. This sequence of casing perforation, fracturing, propping, and perforation plugging is then repeated at successively higher spaced locations along the well casing. When the fracing operation is complete, the perforation plug structures previously formed are removed in a suitable manner to permit enhanced fluid flow from the fracture zones into the casing, through the now unblocked perforation zones therein, for delivery up to the surface-disposed well head in the usual manner.
Critical to the success of this sequential fracturing process is the efficient and reliable formation of the perforation plugging structures at spaced intervals within the casing after the proppant fluid has been delivered to the fracture spaces. In the past, various attempts have been made to form, and precisely locate, these perforation plugging structures within the well casing by flowing a column of perforation plugging slurry downwardly through the casing directly above the fracture proppant fluid and directly beneath a column of driving fluid.
The theory behind this "stacked column" approach to fracture propping and perforation plugging is that after the proppant fluid, and a portion of the perforation plugging slurry, has flowed outwardly through the casing perforations, the plugging slurry will block the perforations and form a casing "plug" structure which extends a short distance upwardly past the upper end of the perforation zone. This plug structure (if successfully formed) defines, in effect, a new "support bottom" portion of the casing above which a subsequent perforation zone may be formed to continue the sequential fracturing operations along the casing.
Difficulties have been encountered, however, in creating these casing plug structures using stacked casing fluid columns. Specifically, in the process of transporting the plugging slurry down-hole, there has tended to be dilution of the plugging slurry due to mixing thereof with fluids above and below it which resulted in an undesirable and quite unpredictable distribution of the plugging slurry content over a long transition zone within the casing. Such dilution and mixing of the plugging slurry with other well bore fluids creates imprecision and uncertainty about achieving a perforation plug-off, and can cause a premature plug-off prior to the desired fluid displacement outflow volume through the perforations. Alternatively, the plug-off may be delayed until long after the calculated displacement volume outflow. Or, a complete plug-off might not even be effected by the slug of perforation plugging slurry which becomes diluted and dispersed during its transit down the casing.
Most prior attempts to circulate slugs of perforation plugging slurries down a long well casing have resulted in failure to achieve a plug-off or, more often, have produced gross inaccuracies in the volumetric displacement of plugging slurries so that the volumetric displacement position of effective plug-off is not achieved. Also, subsequent fall-out of the bypassed solids from the lower slurries, when mixed with the upper fluids, has tended to create an unpredictable casing fill-up of settled-out solids on top of the perforation plug. This excessive casing fill-up from bypassed solids has often made it impossible for the next perforating gun run in the hole to reach the target zone for the next perforation.
These undesirable results stem primarily from the fact that during the flow of the initially stratified slurry and fluid columns down a long casing string, the center of each fluid column is flowing at a much higher velocity than the periphery of the fluid column in the shear zone near the casing wall. Consequently, the fluid near the casing wall has the composition of the fluid from lower down in the column stack. Conversely, the fluid near the center of the casing has the composition of the fluid from higher up in the column stack.
The fluid moving at the higher velociy along the center core of a given fluid column is rushing ahead to invade the next lower slug of fluid. That lower slug of fluid being invaded at the center core is also being retarded by the shearing forces near the casing wall so that it gets strung out along the casing wall through a long transition or mixing zone. Therefore, some of the perforation plugging slurry reaches the target perforation zone in a very diluted form far ahead of its predicted displacement according to time and volume calculations. Likewise, the fluid from the lower segments of the fluid column stack gets strung out along the casing wall for long distances. This tends to greatly dilute the perforation plugging slurry for a considerable distance above and below its calculated displacement position.
To fully appreciate the problems presented in accurately and reliably forming appropriate casing plug structures at each successive perforation zone along the well casing, it must be realized that the plugging "target" is an eight to ten foot perforated casing section located many hundreds or thousands of feet down the well casing. To reliably hit and plug this perforation target, without interfering with the placement of the next perforation gun, requires that a precisely measured amount of plugging slurry be sent down-hole and then caused to interact with the perforation zone in just the right manner.
Conventional attempts to perform this down-hole task have been noticeably less than satisfactory. It is accordingly an object of the present invention to provide an improved method of more reliably and accurately forming these casing plug structures.